Summary: MyRocks wants to be able to un-lock a key that was just locked by GetForUpdate().  To do this safely, I am now keeping track of the number of reads(for update) and writes for each key in a transaction.  UndoGetForUpdate() will only unlock a key if it hasn't been written and the read count reaches 0.

Test Plan: more unit tests

Reviewers: igor, rven, yhchiang, spetrunia, sdong

Reviewed By: spetrunia, sdong

Subscribers: spetrunia, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D47043